As one of the stabilization controls of a grid, frequency regulation which keeps the frequency of the grid within a predetermined range is known. The frequency regulation is a control method of keeping the frequency of the grid within a predetermined range by increasing the output of a power generator which provides power to the power system when the frequency of the grid is lower than the reference frequency, and by decreasing the output of a power generator which provides power to the power system when the frequency of the grid is higher than the reference frequency.
When a power system operator, and a power service provider which has a power generator such as a gas turbine and a steam turbine are different from each other, a method is performed in which the power system operator calculates a power instruction for frequency regulation and the power service provider performs frequency regulation by controlling the output of the power generator based on the instruction.
Recent years have seen the practical use of large-scale power storage apparatuses using a secondary battery or a flywheel capacitor. Although these power storage apparatuses have a relatively small output, they are capable of responding to an output instruction faster and more correctly than the conventional power generation apparatus. In view of this, these power storage apparatuses are expected to stabilize a change in short period frequency of the power system within 10 minutes.
Already in the United States and the like, there is a market which operates power storage apparatuses owned by power service providers through bids. Through the market, the stabilization of the power grid is realized. Moreover, this market introduces a mechanism in which when an apparatus has a higher responsiveness of the output to a power instruction value, a higher incentive is provided.
Different from the power generation apparatus, the power storage apparatus discharges from the power storage apparatus to the grid when the frequency of the grid decreases, and charges from the grid to the power storage apparatus when the frequency of the grid increases. Here, generally, in the frequency regulation, a power value of the power to be charged and discharged by the power storage apparatus is determined by a power instruction value which is transmitted from the power system operator to the power storage apparatus in a period of several seconds, and the power instruction value is determined by the power system operator.
In view of this, the power storage apparatus needs to previously store electricity in order to discharge according to the power instruction value. The power storage apparatus needs to previously secure an electric storage capacity in order to charge according to the power instruction value.
In the frequency regulation, however, there is a case where the power storage apparatus cannot continue charge or discharge when performing charge or discharge according to the power instruction value (for example, Patent Literature 1).
For example, when an average of the power instruction value over a medium to long-term period from the power system operator is ±0, the state of charge (SOC) after the end of the frequency regulation should almost match with the SOC at the start of the frequency regulation. At this time, however, since a power conversion loss and the like occur in an inverter and the like when charge or discharge of power is performed, a discharge amount of the power storage apparatus is larger than a charge amount of the power storage apparatus over a medium to long-term period. In other words, when the power storage apparatus is charged and discharged according to the power instruction value, the SOC decreases as time passes. This means that the power storage apparatus is not able to continue charge or discharge.